1. Materials Science and Engineering, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, USA
2. Department of Electrical & Computer Engineering, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, USA
3. Center for Memory and Recording Research, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0401, USA